The present invention relates to an electromagnetic flowmeter using a square wave excitation current and, more particularly, to an improvement in a means for generating the excitation current.
A conventional electromagnetic flowmeter using a square wave excitation current measures the flow rate of a fluid to be measured in the following manner.
An excitation circuit applies a square wave excitation current to an excitation coil. The magnetic flux generated by the excitation coil is applied to the conductive fluid to be measured. An electromotive force is induced in the fluid in proportion to the magnetic flux and the flow velocity of the fluid, and is detected by a pair of electrodes. When the magnetic flux is stabilized, i.e., immediately before the direction of the magnetic flux is changed, the detected electromotive force is sampled. The average flow rate of the fluid is calculated based upon the sample value.
In the electromagnetic flowmeter with the above arrangement, the electromotive force detected by the electrodes is sampled twice in each cycle of the excitation current. A difference between two time-sequentially adjacent sample values is then generated. If such a sampling method is used, a noise component (e.g., a signal of frequency 2n times of a frequency [excitation frequency] of the excitation current [n is a positive integer], or a signal of a frequency considerably lower than the excitation frequency) can be removed from the signal representing the flow rate. Since the induction noise component of a commercial power source normally has the largest influence, the excitation frequency is set to be 1/2n (mainly, 1/8: n is a positive integer) of the frequency of the commercial power source.
The electrode pair of the electromagnetic flowmeter having the above arrangement is brought into direct contact with the fluid to be measured. For this reason, an electromotive force not associated with one corresponding to the flow rate of the fluid is generated at the electrode pair due to the electrochemical effect of the surfaces of the electrodes and ions contained in the fluid. This electromotive force is called an electrochemical noise. The voltage level of the electrochemical noise is very high, e.g., 10 to 100 times that of the electromotive force corresponding to the flow rate of the fluid. However, the noise normally is a lower frequency than the excitation frequency. For this reason, when sampling is performed, the electrochemical noise can be removed.
The electrochemical noise is largely influenced by the surface state of the electrodes. For example, the noise is changed by slight movement of a fluid in accordance with abrupt pressure variation in a pipe coupled to the electromagnetic flowmeter.
The abrupt pressure variation in the pipe often occurs when a piston pump is used when fluid is not flowing. The piston pump has a normal operation range of about 0.1 to 3 Hz. However, in a double-acting multicylinder pump and the like, the delivery pressure waveform has a frequency component 4 times the pump rotational speed. For this reason, the electrochemical noise has the same frequency component as that of the delivery pressure, and the frequency of the electrochemical noise undesirably coincides with that of the excitation current.
When the frequency of the electrochemical noise coincides with that of the excitation current, the electromagnetic flowmeter operates erroneously. Even if fluid is not flowing, the electromagnetic flowmeter generates a flow rate output, or the flow rate output therefrom is varied, thus preventing normal operation of the flowmeter. In order to overcome this problem, a buffer tank is provided at the delivery side of the pump so as not to change the pressure. However, this method increases cost and requires more space. Alternatively, the excitation frequency can be increased. However, the operation frequency range of the pump is very wide. For this reason, it is difficult to determine the frequency of the electrochemical noise, and the excitation frequency cannot be controlled precisely. When an electromagnetic flowmeter is operated by a square wave excitation current having a high frequency, the response characteristic of the magnetic circuit of the detector must be improved. For this reason, peripheral equipment requires higher precision and power. This increases cost and power consumption of the entire electromagnetic flowmeter.